How to Choose the Right Solar System for Your Boat: A Step-by-Step Guide

Alex Montanari
Aug 17
4 min read

Energy independence at sea is one of the greatest dreams of any sailor. Having enough power from solar panels and batteries means being able to enjoy long passages or anchoring without relying on the engine or marinas.

In this guide, we’ll walk through how to calculate energy consumption, size your batteries correctly, choose the right solar panels, select the best charge controller, and complement the system with a DC-DC charger to ensure you always have the power you need.

1. Calculate the Daily Energy Consumption of Your Devices

The first step is to estimate how much energy your boat consumes in a typical day. To do this, list all onboard appliances (fridge, lights, instruments, pumps, autopilot, etc.) and calculate their daily energy use simply by multiply its wattage by the number of hours it is used daily.

Formula for each device:

Energy (Wh) = Power (W) × Hours of use Energy

Power (W): the device’s instantaneous consumption (e.g., a fridge of 60 W).
Hours of use: the daily average usage time.
Result (Wh): the daily energy consumption of that device.

Repeat this for each device and then add up all the results to get your total daily energy consumption in Wh.

Example:

Fridge: 60 W × 8 h = 480 Wh
LED lights: 20 W × 5 h = 100 Wh
Autopilot: 50 W × 6 h = 300 Wh

Total daily consumption: 480 + 100 + 300 = 880 Wh

2. Convert Consumption into Ampere-Hours (Ah)

Since batteries are rated in Ah, we need to convert Wh into Ah.

Ampere-Hours (Ah) = Energy (Wh) ÷ Electric tension (V)

For a 12V system:

880 Wh ÷ 12 V = 73 Ah/day

3. Battery Bank Sizing

To size the battery bank:

Battery Capacity (Ah) = Daily Consumption (Ah) × Days of Autonomy ÷ Depth of Discharge

Days of autonomy: how many days you want to last without charging (often 2–3 days).
Depth of discharge (DoD):
- Lead-acid: use 50% of capacity.
- Lithium: use 80–90%.

Example with Lithium:

73 Ah/day × 2 days ÷ 0.8 = 182 Ah battery bank

When calculating the size of your battery bank, it’s strongly recommended to choose a capacity larger than the minimum you’ve calculated. This is because batteries should never be discharged too deeply if you want them to last.

A good rule of thumb is to never discharge your battery bank below 80% of its total capacity. For example, if your daily consumption is 200Ah, instead of installing exactly a 200Ah battery bank, it’s better to size it at least 250–300Ah.

This margin ensures:

Longer battery lifespan (avoiding stress from deep discharges)
A safety buffer during cloudy days or unexpected higher consumption
Greater reliability when sailing long distances without shore power

Investing in a slightly larger battery bank means more peace of mind and longer-lasting energy autonomy.

4. Solar Panel Sizing

Now let’s size the solar array needed to recharge the batteries.

Panel Power (W) = Daily Consumption (Wh) ÷ Average Sun Hours

If we have 880 Wh/day and expect 5 peak sun hours:

880 ÷ 5 = 176 W

Add a 20–30% safety margin → you need about 220–250 W of solar panels.

5. Types of Solar Panels: Which One for a Boat?

Not all panels are the same. Different technologies affect efficiency, cost, and durability in the marine environment.

Monocrystalline (Mono-Si):
- ✔ High efficiency (18–22%) → more power in limited space.
- ✔ Good performance in partial shading.
- ❌ Higher cost.
- 👉 Best choice for most boats.
Polycrystalline (Poly-Si):
- ✔ Lower cost.
- ❌ Lower efficiency (15–18%).
- 👉 Suitable only if you have plenty of space.
Flexible panels:
- ✔ Lightweight, can be mounted on biminis, decks, or curved surfaces.
- ✔ Resistant to salt and marine conditions.
- ❌ Lower efficiency and shorter lifespan.
- 👉 Great where rigid panels cannot fit.
Rigid tempered glass panels:
- ✔ Long-lasting, robust, and efficient.
- ❌ Bulkier and heavier.
- 👉 Perfect for dedicated mounts (stern arch, davits, rails).

6. Charge Controllers: PWM vs MPPT

Solar panels must always be connected through a charge controller to regulate the flow of energy into the batteries.

🔹 PWM (Pulse Width Modulation)

✔ Simple and inexpensive.
❌ Only efficient if panel voltage ≈ battery voltage.
❌ Lower efficiency (70–80%). Suitable only for very small systems (<100 W).

🔹 MPPT (Maximum Power Point Tracking)

✔ Advanced, highly efficient (95–98%).
✔ Can use higher-voltage panels with lower-voltage batteries.
✔ Extracts maximum energy in varying sunlight. Recommended for any serious marine system (>100 W).

Tip:

Small system, budget tight → PWM.
Larger system (200–600 W or more) → always MPPT.

7. Complementing with a DC-DC Charger

Solar is great, but when sailing long passages or during cloudy days, it’s useful to also recharge from the engine alternator.

A DC-DC charger (like the Victron Orion Charger series):

Optimizes alternator charging, protecting both engine and house batteries.
Ensures correct charging profile (ideal for lithium).
Works even at anchor or under sail, when solar may not be enough.

This way, every hour of motoring or sailing with the alternator running contributes to your energy independence.

There are also other charging sources available to complement solar, such as shore power chargers, wind or hydro generators or diesel generators.

Conclusion

Designing the right solar system means:

Calculating your real energy needs.
Properly sizing batteries and panels.
Choosing the right type of solar panel for your boat.
Using an MPPT controller for maximum efficiency.
Adding a DC-DC charger for reliable backup charging.

With this setup, your boat will truly be energy independent and ready for long adventures.

At E-Boats Systems, we specialize in designing electrical systems and fully customized solutions for every boat. We can create a complete power management system capable of handling solar and any other charging sources, as well as managing your batteries — fully digitalized also with remote control and monitoring.

👉 Contact us to design your custom system.